Process for preparing aldehydes and cooling a stream of matter

ABSTRACT

A process for preparing aldehydes by a homogeneously catalyzed hydroformylation of C4 to C20 olefins involves withdrawing a biphasic stream (liquid/gaseous) and expanding in two stages. Before, between, or after the two stages, the liquid phase is cooled. Only after expansion and cooling is the homogeneously dissolved rhodium catalyst system separated from the residual stream in a three-stage removal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to European Application No. 21209276.1,flied on Nov. 19, 2021, the content of which is hereby incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a process for preparing aklehydes by ahomogeneously catalyzed hydroformylation of C₄ to C₂₀ olefins, in whicha biphasic stream (liquid/gaseous) is withdrawn and expanded in twostages. Before those, between those or after those, the liquid phase iscooled. Only after expansion and cooling is the homogeneously dissolvedrhodium catalyst system separated from the residual stream in athree-stage removal step.

Description of Related Art

The separation of homogeneously dissolved rhodium catalyst systems fromthe hydroformylation mixtures obtained is a demanding technical problemwhich is solved in the chemical industry, for example, by an evaporationof the crude product stream and/or an organophilic nanofiltration. Whatis common to the two separation processes is that they are generallyconducted at distinctly lower pressures than the precedinghydroformylation reaction itself. Since the hydroformylation istypically conducted with a significant excess of synthesis gas, at leasta portion of the synthesis gas will dissolve in the reaction mixture.Therefore, upstream of the separation processes may be disposed, forexample, an expansion of the reactor output obtained from thehydroformylation, in order to remove dissolved synthesis gas (see WO2014/131623 A1 and the overview of the known prior art therein).

Proceeding from the known prior art, a constant problem is that ofoptimizing the hydroformylation and the downstream process steps. A mainfocus of attention here is on the separation of the rhodium catalystsystem from the reaction mixtures because rhodium is a very costlyprecious metal and losses should be avoided if at all possible. Suchcatalyst losses may occur at various points in the process, for examplein the membrane separation according to the already cited WO 2014/131623A1. The fact that certain amounts of catalysts can also be lost at otherpoints in the process has to date played only a minor role in the priorart.

The problem addressed by the present invention was therefore that ofproviding a process for the hydroformylation of olefins, in which asuitable workup of the reaction mixtures obtained results in lowerlosses of the rhodium catalyst system than in processes known from theprior art.

This problem was solved by the process described in accordance with theinvention for preparing aldehydes by hydroformylation of olefins asdescribed below. Preferred configurations are also specified below.

The process according to the invention is accordingly a process forpreparing aldehydes by hydroformylation of olefins, wherein the processcomprises the following steps:

a) hydroformylating C₄ to C₂₀ olefins, preferably C₆ to C₁₂ olefins, ina reaction zone in the presence of synthesis gas and a homogeneousrhodium catalyst system at a pressure of 100 to 350 bar and atemperature of 90° C. to 250° C., preferably 110° C. to 250° C., toobtain a biphasic hydroformylation mixture, i.e. one comprising a gasphase and a liquid phase, containing at least unconverted olefins,synthesis gas, the homogeneous rhodium catalyst system and the aldehydesformed, which is withdrawn from the at least one reaction zone;

b) firstly expanding the hydroformylation mixture to a pressure between11 bar and 50 bar, and separating the hydroformylation mixture into afirst gaseous phase especially comprising the synthesis gas, and a firstliquid phase especially comprising the unconverted olefins, thehomogeneous rhodium catalyst system and the aldehydes formed;

c) secondly expanding the first liquid phase obtained from step b) to apressure between 1 bar and 10 bar, and separating it into a secondgaseous phase especially comprising the synthesis gas, and a secondliquid phase especially comprising the unconverted olefins, thehomogeneous rhodium catalyst system and the aldehydes formed; and

d) feeding the second liquid phase to an at least three-stage removal,wherein. In the first stage, a first membrane separation is effected, inwhich the rhodium catalyst system is enriched in the retentate and thepermeate is directed to the next stage;

in the second stage, the permeate from the first membrane separation issubjected to a thermal separation in which at least a portion of thealdehydes formed is removed together with the top product, and theliquid bottom product is directed to the next stage; and

in the third stage, the liquid bottom product from the thermalseparation is subjected to a second membrane separation in which therhodium catalyst system is enriched in the retentate, characterized inthat

the biphasic hydroformylation mixture prior to the first expansion instep b), the first liquid phase prior to the second expansion in stepc), or the second liquid phase prior to the feeding to a removal in stepd) is cooled to a temperature between 40 and 100° C.

The invention also includes the following embodiments:

-   1. Process for preparing aldehydes by hydroformylation of olefins,    wherein the process comprises the following steps:    -   a) hydroformylating C₄ to C₂₀ olefins, preferably C₆ to C₁₂        olefins, in a reaction zone in the presence of synthesis gas and        a homogeneous rhodium catalyst system at a pressure of 100 to        350 bar and a temperature of 90° C. to 250° C. to obtain a        biphasic hydroformylation mixture, i.e. one comprising a gas        phase and a liquid phase, containing at least unconverted        olefins, synthesis gas, the homogeneous rhodium catalyst system        and the aldehydes formed, which is withdrawn from the at least        one reaction zone;    -   b) firstly expanding the hydroformylation mixture to a pressure        between 11 bar and 50 bar, and separating the hydroformylation        mixture into a first gaseous phase especially comprising the        synthesis gas, and a first liquid phase especially comprising        the unconverted olefins, the homogeneous rhodium catalyst system        and the aldehydes formed;    -   c) secondly expanding the first liquid phase obtained from        step b) to a pressure between 1 bar and 10 bar, and separating        it into a second gaseous phase especially comprising the        synthesis gas, and a second liquid phase especially comprising        the unconverted olefins, the homogeneous rhodium catalyst system        and the aldehydes formed; and    -   d) feeding the second liquid phase to an at least three-stage        removal, wherein, in the first stage, a first membrane        separation is effected, in which the rhodium catalyst system is        enriched in the retentate and the permeate is directed to the        next stage;    -   in the second stage, the permeate from the first membrane        separation is subjected to a thermal separation in which at        least a portion of the aldehydes formed is removed together with        the top product, and the liquid bottom product is directed to        the next stage; and    -   in the third stage, the liquid bottom product from the thermal        separation is subjected to a second membrane separation in which        the rhodium catalyst system is enriched in the retentate,        characterized in that    -   the biphasic hydroformylation mixture prior to the first        expansion in step b), the first liquid phase prior to the second        expansion in step c), or the second liquid phase prior to the        feeding to a removal in step d) is cooled to a temperature        between 40 and 100° C.-   2. Process according to embodiment 1, wherein the cooling is    conducted in two steps, of which the first cooling takes place after    the hydroformylation in step a) and before the first expansion in    step b) and the second cooling takes place after the second    expansion in step c) and before the removal in step d).-   3. Process according to embodiment 1 or 2, wherein the    hydroformylation in step a) is effected at a pressure between 175    and 300 bar, preferably 200 to 280 bar.-   4. Process according to any of the preceding embodiments, wherein    the hydroformylation in step a) is effected at a temperature in the    range from 120 to 200° C., preferably 120 to 180° C.-   5. Process according to any of the preceding embodiments, wherein    the olefin used in the hydroformylation also functions as solvent.-   6. Process according to any of the preceding embodiments, wherein    the catalyst system used in the hydroformylation comprises not only    rhodium but also a phosphorus-containing ligand.-   7. Process according to any of the preceding embodiments, wherein    the expansion in step b) and/or in step c) is into a vessel in which    gas phase and liquid phase can be separated.-   8. Process according to any of the preceding embodiments, wherein    the cooling is effected by means of a discharge cooler.-   9. Process according to any of the preceding embodiments, wherein    the membrane separations in step d) are effected at a temperature    between 20 and 80° C., preferably 40 to 70° C.-   10. Process according to any of the preceding embodiments, wherein    the transmembrane pressure in the membrane separations in step d) is    between 15 and 50 bar, preferably between 20 and 45 bar.-   11. Process according to any of the preceding embodiments, wherein    the rhodium catalyst system is obtained in the retentate in the    membrane separations in step d) and the retentate from the first    membrane separation is returned to the reaction zone in step a) and    the retentate from the second membrane separation is recycled    upstream of the first membrane separation in step d).-   12. Process according to any of the preceding embodiments, wherein    the thermal separation is a distillation, a thin-film evaporation, a    falling-film evaporation or a combination of two or more of these.-   13. Process according to embodiment 12, wherein the thermal    separation is a thin-film evaporation, a falling-film evaporation or    a combination of thin-film evaporation and falling-film evaporation.-   14. Process according to embodiment 11 or 13, wherein the thermal    separation is multistage falling-film evaporation.-   15. Process according to any of the preceding embodiments, wherein    the aldehydes obtained from the process are sent to a downstream    process step, for example a hydrogenation or an oxidation.

DETAILED DESCRIPTION OF THE INVENTION

The process according to the invention has the advantage that thecooling prior to the expansion can demonstrably reduce rhodium loss. Theprocess according to the present invention accordingly offerssignificant economic benefits since rhodium is very costly. It also hasenvironmental benefits because less rhodium precipitates out and has tobe removed and disposed of in a costly and inconvenient manner.Furthermore, the process is advantageous owing to the two-stageexpansion. The two-stage expansion prevents additional precipitation ofrhodium as a result of an excessive and excessively rapid reduction inpressure. Furthermore, the two-stage expansion prevents foaming in thereaction solution to be expanded. Foaming leads to problems in thedownstream process steps.

In the first step a), the C₄ to C₂₀ olefins, preferably C₈ to C₁₂olefins, used are first hydroformylated. The hydroformylation takesplace here as usual in the presence of synthesis gas (a mixture of COand H₂) and a homogeneously dissolved rhodium catalyst system.

Suitable hydrocarbon streams are typically used for the provision of theolefins in the process according to the invention. The C₄ to C₂₀olefins, preferably C₈ to C₁₂ olefins, envisaged in accordance with theinvention for the hydroformylation may be olefins having terminal and/orinternal C—C double bonds. The hydrocarbon streams mentioned may alsocomprise olefins having the same number or different numbers of carbonatoms. Suitable olefins are especially 1- or 2-butene or mixturesthereof, isobutene, 1- or 2-pentene or mixtures thereof, isopentenes,1-, 2- or 3-hexene, 1-heptene, linear heptenes with an internal doublebond (2-heptene, 3-heptene, etc.), mixtures of linear heptenes, 2- or3-methyl-1-hexene, 1-octene, linear octenes with an internal doublebond, mixtures of linear octenes, 2-, 3- or 5-methylheptenes,2-ethylhexenes, 2-ethyl-3-methylpentenes, 3,4-dimethylhexenes, ormixtures of the aforementioned linear and branched C₈ olefins (calleddi-n-butene), 2,4,4-trimethylpentenes (called diisobutene), mixtures ofdi-n-butene and diisobutene, 1-nonene, linear nonenes with an internaldouble bond, mixtures of linear nonenes, 2-, 3- or 4-methyloctenes, 1-,2-, 3-, 4- or 5-decene, 2-ethyl-1-octene, 1-dodecene, linear dodeceneswith an internal double bond, branched dodecenes (meaning branchedolefins having 12 carbon atoms) with a terminal or internal double bond,mixtures of such linear and branched dodecenes (called tri-n-butene),1-tetradecene, linear tetradecenes with an internal double bond,mixtures of linear tetradecenes, 1-hexadecene, linear hexadecenes withan internal double bond and mixtures of linear hexadecenes.

C₅ olefins, i.e. pentenes, are present in light petroleum fractions fromrefineries or crackers. Industrial mixtures comprising C₄ olefins, i.e.n-butene and isobutene, are light petroleum fractions from refineries,C₄ fractions from FC crackers or steam crackers, mixtures fromFischer-Tropsch syntheses, mixtures from dehydrogenation of butanes, andmixtures resulting from metathesis or other industrial processes. Thehigher olefins can in particular be obtained by oligomerizationreactions, for example dimerization, trimerization or tetramerization.Suitable hydrocarbon streams are in addition the mixture of isomerichexenes (dipropene) resulting from the dimerization of propene, themixture or isomeric octenes (dibutene) resulting from the dimerizationof butenes, the mixture of isomeric nonenes (tripropene) resulting fromthe trimerization of propene, the mixture of isomeric dodecenes(tetrapropene or tributene) resulting from the tetramerization ofpropene or the trimerization of butenes, the isomeric hexadecene(tetrabutene) resulting from the tetramerization of butenes and also theolefin mixtures produced by the co-oligomerization of olefins having avarying number of carbon atoms (preferably 2 to 4 carbon atoms),optionally after distillative separation into fractions having the sameor different numbers of carbon atoms. Olefins or olefin mixturesproduced by Fischer-Tropsch synthesis may also be used. It isadditionally possible to use olefins produced by olefin metathesis or byother industrial processes.

The olefins used in the process are hydroformylated with synthesis gas.The synthesis gas. i.e. a mixture of carbon monoxide and hydrogen, forthe process according to the invention may be used in different mixingratios of carbon monoxide and hydrogen. The molar ratio betweensynthesis gas and the hydrocarbon stream used that contains the olefinsto be hydroformylated should be between 6:1 and 1:1, preferably between3:1 and 1:1, more preferably between 2:1 and 1:1. The hydroformylationcan optionally be conducted in the presence of an additional solventknown to the person skilled in the art, but preference is given toadding no additional solvent, with the olefin used instead functioningas solvent in the hydroformylation.

The homogeneous rhodium catalyst system used in the hydroformylation instep a) of the process according to the invention comprises or consistsof rhodium (Rh) and preferably a phosphorus-containing ligand. Suitableligands for the catalyst systems of the invention are known to thoseskilled in the art (see e.g. the textbooks “Rhodium CatalyzedHydroformylation” (from 2002) by P.W.N.M. van Leeuwen or“Hydroformylation—Fundamentals, Processes and Applications in OrganicSynthesis” (from 2016) by A. Börner and R. Franke). Thephosphorus-containing ligand for the catalyst system of the invention ispreferably a phosphine (e.g. TPP (triphenylphosphine)), a monophosphite(e.g. Alkanox 240 (tris(2,4-di-tert-butylphenyl)phosphite)) or abisphosphite (e.g. BiPhePhos). It is also possible to use mixtures ofligands. The rhodium concentration in the reaction zone in step a) Ispreferably between 10 and 80 ppm, more preferably between 30 and 50 ppm.

In a preferred embodiment of the present invention, the rhodium catalystsystem, prior to entry into the reaction zone, is dissolved in at leasta portion of the solvent, i.e., for example, the olefins used or thehydrocarbon stream used that contains the olefins used, and then fed tothe reaction zone. This has the advantage that the catalyst systemalready arrives in dissolved form in the reaction zone and need not bedissolved therein before it can catalyze the hydroformylation. Theresulting solution of rhodium catalyst system in olefin/solvent may bepreheated prior to entry into the reaction zone. As a result, there isno need to use any energy for the heating in the reaction zone.

The hydroformylation in step a) of the process according to theinvention is conducted under the following conditions: The temperaturein the hydroformylation is within a range from 90° C. to 250° C.,preferably 110° C. to 250° C., more preferably within a range from 120to 200° C. and particularly preferably within a range from 120 to 160°C. The pressure in the hydroformylation is within a range from 100 to350 bar, preferably within a range from 175 to 300 bar and morepreferably within a range from 200 to 280 bar.

The hydroformylation is according to the invention carried out in atleast one reaction zone. A reaction zone for the purposes of the presentinvention comprises at least one reactor in which the hydroformylationis carried out. It is also possible for the reaction zone to comprisemore than one reactor, in particular two or three reactors, which can beconnected in parallel or in series or arranged in a hybrid of paralleland serial connection.

In the hydroformylation in step a), according to the invention, abiphasic hydroformylation mixture, i.e. one comprising a gas phase and aliquid phase, which contains at least unconverted olefins, synthesisgas, the homogeneous rhodium catalyst system and the aldehydes formed,is obtained. In this case, the synthesis gas is both in dissolved form,i.e. dissolved in the liquid organic phase, and in the form of freesynthesis gas, i.e. the gas phase of the biphasic hydroformylationmixture is formed. The hydroformylation mixture may additionally containhigh-boiling compounds that can form as by-products in thehydroformylation.

In step b), the optionally cooled hydroformylation mixture obtained isexpanded from the present reaction pressure to a pressure between 11 barand 50 bar, preferably in a suitable vessel in which gas phase andliquid phase can be separated. One example of this is an expansionvessel known from the prior art. This first expansion is preferablyconducted in a single step, but may also be effected in multiplecomponent steps. It may also be advantageous when, for each of thecomponent expansion steps, there is at least one vessel for separationof the first liquid phase from the gaseous phase.

The first expansion converts dissolved synthesis gas to the gas phase.Step b) therefore also includes the separation of the hydroformylationmixture into a first gaseous phase comprising synthesis gas to apredominant degree, i.e. to an extent of more than 90% by volume, and afirst liquid phase comprising the unconverted olefins, the homogeneousrhodium catalyst system and the aldehydes formed. It is also possiblefor still-dissolved synthesis gas to be present in the first liquidphase. If high-boiling compounds are additionally also present in thehydroformylation mixture, these high-boiling compounds are likewisepresent in the first liquid phase.

The first gaseous phase separated off in the separation in step b),especially comprising the synthesis gas, may at least partly be returnedto the reaction zone. Recycling is not always desired or required. Itmay therefore be advantageous when the first gaseous phase separated offis sent to another use, for example production or hydrogen. If the firstgaseous phase is returned to the reaction zone, it is preferable thatthe first gaseous phase is compressed before entry into the reactor(s)of the reaction zone. It is particularly preferable here when the firstgaseous phase passes through a cooler beforehand in order to separateoff organic vapours and entrained droplets as condensate. The condensatethus obtained may be guided to the liquid phase from the phaseseparation vessel. It may also be advantageous to free the synthesis gasof impurities, for example of formic acid formed as a by-product in thehydroformylation, by a scrubbing operation prior to further use(recycling or other use). Useful scrubbing media, as well as water andamine-containing aqueous solutions, also include organic solvents. Inpractice, it has been found to be advantageous to use the high boilersobtained as by-products in the hydroformylation as scrubbing liquid.

The first liquid phase obtained from the separation in step b) isdirected into step c) for the second expansion. In this step c), thefirst liquid phase obtained is expanded from the present pressure fromthe first expansion to a pressure between 1 bar and 10 bar, preferablyin a suitable expansion vessel. In principle, the pressure attained hereshould be guided by the permeate pressure of the first membraneseparation in step d); in particular, the pressure should be somewhatlower than this permeate pressure. This second expansion is preferablyconducted in a single step, but may also be effected in multiplecomponent steps. It may also be advantageous when, for each of thecomponent expansion steps, there is at least one expansion vessel forseparation of the liquid phase from the gaseous phase.

The second expansion can also convert still-dissolved synthesis gas tothe gas phase. The second expansion in step c) therefore also includesthe separation of the hydroformylation mixture into a second gaseousphase comprising synthesis gas to a predominant degree, i.e. to anextent of more than 90% by volume, and a second liquid phase comprisingthe unconverted olefins, the homogeneous rhodium catalyst system and thealdehydes formed. Small amounts of dissolved synthesis gas mayadditionally be present in the second liquid phase. If high-boilingcompounds are additionally also present in the hydroformylation mixture,these high-boiling compounds are likewise present in the first liquidphase.

The second gaseous phase separated off in the separation in step c),especially comprising the synthesis gas, may at least partly be returnedto the reaction zone. Recycling is not always desired or required. Itmay therefore be advantageous when the second gaseous phase separatedoff is sent to another use, for example production of hydrogen orincineration. If the second gaseous phase is returned to the reactionzone, it is preferable that the second gaseous phase is compressedbefore entry into the reactor(s) of the reaction zone. It isparticularly preferable here when the second gaseous phase passesthrough a cooler beforehand in order to separate off organic vapours andentrained droplets as condensate. The condensate thus obtained may beguided to the first and/or second liquid phase from the phase separationvessel.

The second liquid phase obtained from the second expansion in step c) isthen guided to an at least three-stage removal in step d).

The characterizing feature of the present invention is the cooling ofthe stream at a point in the process, but no later than before theremoval in step d). The cooing of the stream ensures a reduction in therhodium use factor. The problem is that a small portion of the rhodiumis always lost in various ways during a hydroformylation and thesubsequent removal. On account of the high costs of rhodium or rhodiumcompounds, this increases the process costs, because the losses ofrhodium have to be compensated for by replenishment. However, thecooling according to the invention has the effect that the use factorfalls, i.e. less rhodium is lost and less rhodium also has to bereplenished. The process costs can thus be lowered considerably in thisway.

In the present context, it is less critical when the cooling iseffected, provided that the cooling follows the hydroformylation in stepa) and precedes the removal in step d). For instance, the cooing of thebiphasic hydroformylation mixture can be conducted before the firstexpansion in step b), on the first liquid phase before the secondexpansion in step c), or on the second liquid phase before the feedingto the removal in step d). In a preferred embodiment of the presentinvention, the cooling takes place on the biphasic hydroformylationmixture, i.e. after the hydroformylation in step a) and before the firstexpansion in step b). In the cooing, the respective liquid phase to becooled or the hydroformylation mixture is cooled to a temperaturebetween 40 and 100° C., preferably between 50 and 95° C., morepreferably between 60 and 90° C. For this purpose, a suitable coolingapparatus is required, especially for biphasic substance mixtures, whenthe biphasic hydroformylation mixture is to be cooled. The cooing isespecially effected with a discharge cooler. Shell and tube heatexchangers, for example, have been found to be useful, wherein thereaction mixture is preferably guided through the tubes and the coolingmedium preferably through the shell of the heat exchanger.

In a further-preferred embodiment of the present invention, the coolingis conducted in two steps. A first cooling operation takes place here onthe biphasic hydroformylation mixture, i.e. after the hydroformylationin step a) and before the first expansion in step b), and a secondcooling operation on the second liquid phase, i.e. after the secondexpansion in step c) and before the removal in step d). In the firstcooing operation, the mixture present is preferably cooled to atemperature between 80 and 90° C. The second cooling operationpreferably involves cooling to a temperature between 35 and 55° C.Preference is given here to the use of a discharge cooler for the twosteps. Shell and tube heat exchangers, for example, have been found tobe useful, wherein the reaction mixture is preferably guided through thetubes and the cooling medium preferably through the shell of the heatexchanger.

The removal in step d) comprises at least three stages, wherein, in thefirst stage, a first membrane separation is effected, in which therhodium catalyst system is enriched in the retentate and the permeate isdirected to the next stage; In the membrane separation, the rhodiumcatalyst system is obtained in the retentate, while the aldehydes formedare enriched in the permeate. If high boilers are present, these areobtained at least partly in the permeate. The membrane separation itselfcan be effected at a temperature between 20 and 80° C., preferably 40 to70° C. The transmembrane pressure in the membrane separation in step d)is preferably between 15 and 50 bar, preferably between 20 and 45 bar.In order to arrive at this transmembrane pressure, it may be necessaryfor an actuator to be present, which increases the pressure of themixture guided to the membrane separation. One example is a pump or apressure-increasing pump.

In the second stage of the removal in step d), a thermal separation isperformed, in which at least a portion of the aldehydes formed isremoved together with the top product, and the liquid bottom product isdirected to the next stage.

The thermal separation in the second stage of the removal in step d) maybe a distillation, a thin-film evaporation, a falling-film evaporationor a combination of two or more of these. However, the person skilled inthe art may choose a suitable method in accordance with the demands ofthe respective process on the basis of their knowledge in the art. In apreferred embodiment of the present invention, the thermal separation isa thin-film evaporation, a falling-film evaporation or a combination ofthin-film evaporation and falling-film evaporation. Thin-filmevaporation and falling-film evaporation may also have a multistageconfiguration, i.e. take the form of what is caged a cascade. In aparticularly preferred embodiment, the thermal separation is amultistage falling-film evaporation (evaporator cascade). If a thin-filmevaporation, a falling-film evaporation or a combination of these isused for thermal separation, expansion after the first membraneseparation step must be into a vacuum. i.e. a pressure of ≥10 mbar butless than 1 bar. Such an expansion is preferably effected in one step.

The membrane separation in the third stage of the removal in step d) maybe effected in one or more stages, but is preferably effected in two ormore stages. In the membrane separation, the rhodium catalyst system isobtained in the retentate, while the aldehydes formed are enriched inthe permeate. If high boilers are present, these are at least partlyobtained in the permeate, as a result of which they can be dischargedfrom the process. This is because, in order to prevent enrichment of thehigh boilers in the process, a portion of the permeate or the entirepermeate may be discharged from the process as purge. The rhodiumcatalyst system obtained in the retentate, by contrast, is preferablyreturned upstream of the first membrane separation in step d). Themembrane separation itself can be effected at a temperature between 20and 80° C., preferably 40 to 70° C. The transmembrane pressure in themembrane separation in step d) is preferably between 15 and 50 bar,preferably between 20 and 45 bar.

The aldehydes obtained from the process, as product of value, are usablein various ways in the chemical industry. For instance, these aldehydesmay be used as a starting material. The aldehydes obtained are then sentto a downstream process step, for example a hydrogenation to give thealcohol or an oxidation to give the acid.

EXAMPLES Example 1—Hydroformylation of Tri-n-Butene

In an industrial pilot plant comprising a reactor (cascaded bubblecolumn), a thin-film evaporator and a membrane separation,hydroformylation experiments were conducted with tri-n-butene. For thispurpose, a tri-n-butene stream was reacted with synthesis gas (50%CO/50% H₂) at about 270 bar and about 150° C. in the reactor. Thebiphasic product mixture containing the isotridecanal (ITDA) product,after optional cooling, is then expanded and separated in a thin-filmevaporator (at <50 mbar), with removal of the product-containing phase.The residual stream is fed to a two-stage membrane separation (TMP 40bar, temperature 50° C.), wherein the catalyst-containing retentate isguided to the reactor and the permeate is discharged as purge stream.

The experiment was conducted once with discharge cooling, i.e. coolingprior to the expansion, and once without discharge cooling. In eachcase, the rhodium use factor, i.e. the amount of rhodium in g requiredfor production of one tonne of ITDA, was normalized to the value for theprocess conducted according to the prior art (=without cooling). Theresults can be seen in the table below.

Rhodium use factor (normalized to prior art) with prior cooling of thebiphasic product 0.67 mixture (inventive) without prior cooling of thebiphasic product 1 mixture (prior art)

It is clear from the table that the process can be conducted much moreefficiently and less expensively since the use factor is distinctlysmaller in the process with discharge cooling.

Example 2: Hydroformylation of Diisobutene

In a hydroformylation plant comprising, inter alia, a reactor (cascadedbubble column), a thin-film evaporator and a membrane separation,hydroformylation experiments were conducted with diisobutene. For thispurpose, a diisobutene stream was reacted with synthesis gas (50% CO/50%H₂) at about 270 bar and about 130° C. in the presence of a rhodiumcatalyst system in the reactor. The biphasic product mixture thatcontains the TMH (trimethylhexanal) product, after optional cooling, isthen expanded and separated in the thin-film evaporator (at <50 mbar),with removal of the product-containing phase. The residual stream issent to a one-stage membrane separation (TMP 40 bar, temperature 50°C.), wherein the catalyst-containing retentate is guided to the reactorand the permeate is discharged as purge stream.

The experiment was conducted once with discharge cooling, i.e. coolingprior to the expansion, and once without discharge cooling. In eachcase, the rhodium use factor, i.e. the amount of rhodium in g requiredfor production of one tonne of TMH, was normalized to the value for theprocess conducted according to the prior art (=without cooling). Theresults can be seen in the table below.

Rhodium use factor (normalized to the prior art) with prior cooling ofthe biphasic product 0.685 mixture (inventive) without prior cooling ofthe biphasic product 1 mixture (prior art)

It is clear from the table that the process can be conducted much moreefficiently and less expensively since the use factor is distinctlysmaller in the process with discharge cooling.

1. A process for preparing aldehydes by hydroformylation of olefins, theprocess comprising: a) reacting C₄ to C₂₀ olefins in a hydroformylationin at least one reaction zone in the presence of a synthesis gas and ahomogeneous rhodium catalyst system, at a pressure of 100 to 350 bar anda temperature of 90° C. to 250° C., to obtain a biphasichydroformylation mixture comprising a gas phase and a liquid phase,wherein the biphasic hydroformylation mixture contains at leastunconverted olefins, the synthesis gas, the homogeneous rhodium catalystsystem, and aldehydes formed, which is withdrawn from the at least onereaction zone; b) expanding the biphasic hydroformylation mixture in afirst expansion, to a pressure between 11 bar and 50 bar, and separatingthe biphasic hydroformylation mixture into a first gaseous phasecomprising the synthesis gas, and a first liquid phase comprising theunconverted olefins, the homogeneous rhodium catalyst system, thealdehydes formed, and optionally, remaining synthesis gas; c) expandingthe first liquid phase obtained from b) in a second expansion, to apressure between 1 bar and 10 bar, and separating the first liquid phaseinto a second gaseous phase comprising the remaining synthesis gas, anda second liquid phase comprising the unconverted olefins, thehomogeneous rhodium catalyst system, and the aldehydes formed; and d)feeding the second liquid phase to an at least three-stage removal,wherein, in the first stage, a first membrane separation is effected, inwhich the homogeneous rhodium catalyst system is enriched in a firstretentate and a permeate is directed to the next stage; in the secondstage, the permeate from the first membrane separation is subjected to athermal separation in which at least a portion of the aldehydes formedis removed together with a top product, and a liquid bottom product isdirected to the third stage; and in the third stage, the liquid bottomproduct from the thermal separation is subjected to a second membraneseparation in which the homogeneous rhodium catalyst system is enrichedin a second retentate, wherein the biphasic hydroformylation mixtureprior to the first expansion in b), the first liquid phase prior to thesecond expansion in c), or the second liquid phase prior to the feedingto the at least three-stage removal in d) is cooled to a temperaturebetween 40 and 100° C.
 2. The process according to claim 1, comprisingboth a first cooling of the biphasic hydroformylation mixture after thehydroformylation in a) and before the first expansion in b), and asecond cooling of the second liquid phase after the second expansion inc) and before the at least three-stage removal in d).
 3. The processaccording to claim 1, wherein the hydroformylation in a) Is effected ata pressure between 175 and 300 bar.
 4. The process according to claim 1,wherein the hydroformylation in a) is effected at a temperature in arange from 120 to 200° C.
 5. The process according to claim 1, whereinthe C₄ to C₂₀ olefins used in the hydroformylation also function assolvent.
 6. The process according to claim 1, wherein the homogeneousrhodium catalyst system used in the hydroformylation comprises rhodiumand a phosphorus-containing ligand.
 7. The process according to claim 1,wherein the first expansion in b) and/or the second expansion in c) isinto a vessel in which gas phase and liquid phase can be separated. 8.The process according to claim 1, wherein the cooling is effected by adischarge cooler.
 9. The process according to claim 1, wherein the firstmembrane separation and the second membrane separation in d) areeffected at a temperature between 20 and 80° C.
 10. The processaccording to claim 1, wherein a transmembrane pressure in the firstmembrane separation and the second membrane separation in d) is between15 and 50 bar.
 11. The process according to claim 1, wherein thehomogeneous rhodium catalyst system is obtained in the first retentatein the first membrane separation and the second retentate in the secondmembrane separation in d), and wherein the first retentate from thefirst membrane separation is returned to the at least one reaction zonein a), and the second retentate from the second membrane separation isrecycled upstream of the first membrane separation in d).
 12. Theprocess according to claim 1, wherein the thermal separation is adistillation, a thin-film evaporation, a falling-film evaporation, or acombination of two or more thereof.
 13. The process according to claim12, wherein the thermal separation is the thin-film evaporation, thefalling-film evaporation, or a combination of thin-film evaporation andfalling-film evaporation.
 14. The process according to claim 11, whereinthe thermal separation is a multistage falling-film evaporation.
 15. Theprocess according to claim 1, wherein the aldehydes obtained from theprocess are sent to a downstream process.
 16. The process according toclaim 1, wherein the C₄ to C₂₀ olefins are C₈ to C₁₂ olefins.
 17. Theprocess according to claim 3, wherein the hydroformylation in a) iseffected at a pressure between 200 and 280 bar.
 18. The processaccording to claim 4, wherein the hydroformylation in a) is effected ata temperature in a range from 120 to 160° C.
 19. The process accordingto claim 13, wherein the thermal separation is a multistage falling-filmevaporation.
 20. The process according to claim 15, wherein thedownstream process is a hydrogenation or an oxidation.